1. Field of the Invention
The present invention relates to an AC-DC converter that receives an alternating-current power supply and outputs a direct-current voltage and, in particular, to a power factor correction (PFC) converter that improves the power factor.
2. Description of the Related Art
Japan and Europe, for example, have enforced harmonic current regulations classified according to the application, the input power, and other factors. To address these regulations, circuits called PFC converters have been added to the power supplies of general home electrical appliances to which the regulations apply so as to suppress harmonic current.
A general switching power supply device using a commercial alternating-current power supply as an input power supply rectifies and smoothes the commercial alternating-current power supply to convert it into a direct-current voltage and then switches the direct-current voltage in a DC-DC converter. Thus, the input current is discontinuous and significantly deviates from a sine wave. This results in harmonic current.
To suppress such harmonic current, a PFC converter is disposed between a full-wave rectifier circuit and a smoothing circuit that includes a smoothing capacitor.
This PFC converter includes a chopping circuit and operates so that the input current waveform is similar to the input voltage waveform, that is, the input current waveform has a sinusoidal shape in phase with the input voltage waveform. Thus, harmonic current is suppressed to a certain level or below.
Hereafter, an example configuration of a PFC converter shown in Japanese Unexamined Patent Application Publication No. 2004-282958 will be described with reference to FIG. 1. In a power-factor improvement circuit shown in FIG. 1, a series circuit that includes an inductor L1, a switching element Q1, which is a MOSFET, and a current detection resistor R is connected to both output terminals of a diode bridge B1, which rectifies the alternating-current power supply voltage of an alternating-current input power supply vac. A series circuit that includes a diode D1 and a smoothing capacitor C1 is connected to both end portions of the switching element Q1, and a load RL is connected to both end portions of the smoothing capacitor C1. The switching element Q1 is turned on or off under the PWM control of a control circuit 10. The current detection resistor R detects input current passing through the diode bridge B1.
The control circuit 10 includes an error amplifier 111, a multiplier 112, an error amplifier 113, a voltage control oscillator (VCO) 115, and a pulse width modulation (PWM) comparator 116.
The error amplifier 111 obtains the error between the voltage of the smoothing capacitor C1 and a reference voltage E1. The multiplier 112 multiplies the error voltage signal by the voltage rectified by the diode bridge B1. The error amplifier 113 generates an error between the result of the multiplication performed by the multiplier 112 and the current signal passing through the diode bridge B1 and outputs the error to the PWM comparator 116.
The VCO 115 generates a chopping signal with a frequency corresponding to the rectified voltage value of the alternating-current power supply voltage.
The negative terminal of the PWM comparator 116 receives the chopping signal from the VCO 115, and the positive terminal thereof receives the signal from the error amplifier 113. That is, the PWM comparator 116 provides to the switching element Q1 a duty pulse corresponding to the current passing through the diode bridge B1 and the output voltage. This duty pulse is a pulse width control signal that successively compensates for variations in the alternating-current power supply voltage and the direct-current load voltage in a certain period.
Due to this configuration, control is performed so that the current waveform of the alternating-current power supply is similar to and in phase with the voltage waveform of the alternating-current power supply. Thus, harmonics and the power factor are improved.
On the other hand, Japanese Unexamined Patent Application Publication No. 7-177746 is disclosed as a PFC converter that performs digital control. In addition, with digital control, current passing through the inductor is detected, and the switching element is switched under PWM control corresponding to the value of the current.
Meanwhile, to properly achieve harmonics suppression and power factor improvement, which are the objectives of PFC converters, rapid and highly accurate detection of the current passing through the inductor is required. To detect the operation state of a PFC converter and to perform a process corresponding to the operation state, rapid and highly accurate detection of the current passing through the inductor is also required.
Conventional PFC converters that perform switching control using an analog circuit, such as the PFC converter of Japanese Unexamined Patent Application Publication No. 2004-282958, cannot sufficiently rapidly detect the current passing through the inductor. PFC converters that perform digital control, such as the PFC converter of Japanese Unexamined Patent Application Publication No. 7-177746, must originally perform a large amount of operation processing, and the response speed and accuracy have a tradeoff relationship. For these reasons, there is a limit to the amount by which responsiveness can be improved while maintaining accuracy.